This invention relates generally to surface acoustic wave stabilized oscillators and, more particularly, to surface acoustic wave stabilized oscillators having low vibration sensitivity.
As it is known in the art, surface acoustic wave devices (SAW devices) are employed in a variety of applications, such as resonators and delay lines for oscillator circuits, as well as, filters and pressure transducers. Generally, a SAW device comprises at least one transducer including a set of conductive members which is disposed on or recessed within a surface of a piezoelectric substrate.
In many applications of SAW devices, particularly with respect to applications of resonators and delay lines, as frequency stabilizing and determining elements in oscillators, it is important to provide a package having a relatively small size while, at the same time, properly mounting the SAW device within the package to reduce the so-called vibration sensitivity of the SAW device. It is known that the resonant frequency of an oscillator, including a SAW device, is sensitive of external vibration or changes in external stress applied to the SAW device. The sensitivity results from the external stress on the piezoelectric substrate causing changes in surface wave velocity and hence resonant frequency characteristics of the SAW device. In particular, the surface wave velocity is influenced by applied forces through the second order elastic coefficients of the material of the SAW substrate and also because the physical distance between the transducers or gratings on the substrate is changed. Thus, both of these factors contribute to net frequency change in SAW devices such as delay lines and resonators.
Conventional packages such as the TO-8 package and flatpacks which are hermetically sealed are often used. Such packages are relatively large in volume in comparison to the size of the SAW device. These packages also constrained how the SAW substrate can be mounted within the package to provide a SAW device having a surface wave velocity which is relatively invariant with external stress.
It is generally known that a low vibration sensitivity is obtained when the bottom of the SAW device substrate is uniformally supported. One approach is to provide a soft stress reducing material such as a rubber or a room temperature vulcanizing silicon rubber on the conventional package to uniformally support the bottom of the SAW device substrate. It has been found, however, that such a soft material causes long-term frequency shifts in SAW devices provided within such packages because over time the soft material will outgas impurities which may become deposited upon the upper surface of the SAW device substrate. Such deposits of impurities are believed to cause changes in the velocity characteristics of the surface waves which propagate along the upper surface of the substrate and hence change the long term frequency characteristics of the device. These frequency characteristics are often of a magnitude so large that the device is no longer acceptable for many applications. On the other hand, rigidly fastening the bottom of the SAW substrate to the package is also generally unacceptable since the thermal expansion characteristics of the SAW substrate are generally not perfectly matched to the thermal expansion characteristics of the material of the package. Because of this mismatch in thermal expansion characteristics, this arrangement leads to unpredictable temperature dependent stress characteristics that adversely effect frequency stability and may even result in fracture of the base of the SAW device.
Once solution to the foregoing problem has been to provide a hermetically sealed package in which the SAW substrate provides a bottom portion of the package. Such arrangements are shown in U.S. Pat. No. 4,270,105, Parker, et al., issued May 26, 1981, entitled "Stabilize Surface Wave Device" and assigned to the assignee of the present invention and in articles entitled "Long Term Aging and Mechanical Stability of 1.4 Gigahertz SAW Oscillators" by M. Gliden, et al., Proceedings of the IEEE Ultrasonic Symposium, page 184 and "SAW Resonator Frit Bonded Pressure Transducer" by D. Weirauch, et al., Proceedings of the IEEE Ultrasonic Symposium, 1979, page 874.
The issued U.S. Patent describes a quartz package having the hydrophobic polymer coating for passivating an upper surface of the substrate on which the surface waves propagate. The article by Gliden describes a SAW based oscillator including a quartz packaged SAW device. Long term aging data indicates that such devices will have frequency shifts of at least .+-.4 parts per million per year. For some applications in stable oscillators, this drift or aging characteristic is unacceptable. The second article by Weirauch describes a pressure transducer fabricated having a quartz package. According to this article, a frit was applied to both surfaces, the SAW was evacuated, and the substrates were then mated together. As indicated in the article, a hysteresis effect was observed. This indicates that there may be a stress relief problem associated with the technique. It was also suggested that some long term frequency shift effects may be present. These short term and potential long term drift problems may be unacceptable for SAW devices such as delay lines and resonators when used in highly stable precision oscillators.
A solution to these problems of aging and short term frequency shift has been the All-Quartz Package (AQP) as mentioned in a paper entitled "A New All-quartz Package for SAW Devices" by Parker, et al., 39th Annual Frequency Symposium, 1985, pages 519-524. The approach has been used to provide devices such as resonators having very low aging rates (typically less than 0.5 parts per million per year). The approach has also been used on delay lines. The distinction made between SAW resonators and SAW delay lines is that a SAW resonator generally also has a pair of gratings disposed on the surface wave surface outside of the region of the pair of transducers. The resonator thus provides a high Q narrow band device whereas the SAW delay line does not have such a pair of gratings and is a broaderband, lower Q device.
The vibration sensitivity of SAW devices used in SAW stabilized oscillators is particularly important for radar applications where the SAW stabilized oscillator will be in a high vibration environment, such as, for example, on a plane, helicopter, or missile. Frequency fluctuations induced by external vibrations can cause a significant increase in oscillator phase noise levels at the vibration frequencies and therefore degrade the performance of the radar. Theoretical predictions of the vibration sensitivity for the all-quartz package resonators indicate that vibration sensitivities as low as 10.sup.-11 fractional change in frequency per g of applied force should be attainable in each of the three orthogonal directions of the SAW device. Such performance will be comparable to or better than any known oscillator technology (i.e. bulkwave technologies or dielectric resonator technologies for example). In practice, however, it has not been heretofore possible to provide SAW stabilized oscillations in which the SAW device exhibits such low SAW vibration sensitivities.